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                <h1 class="kratos-entry-title text-center" itemprop="name headline">【Python】常用函数</h1>
            
            
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                <li><time datetime="2023-12-18T16:00:00.000Z" itemprop="datePublished"><i class="fa fa-calendar"></i> 2023-12-19</time></li>
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                    <i class="fa fa-user"></i> 作者 <span itemprop="name">林家隆</span>
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                <div class="text"><p>本文最后编辑于 <time datetime="1763638522355"></time> 前，其中的内容可能需要更新。</p></div>
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                    <ol class="toc"><li class="toc-item toc-level-2"><a class="toc-link" href="#%E5%AD%97%E7%AC%A6%E4%B8%B2"><span class="toc-number">1.</span> <span class="toc-text">字符串</span></a><ol class="toc-child"><li class="toc-item toc-level-3"><a class="toc-link" href="#lower"><span class="toc-number">1.1.</span> <span class="toc-text">lower()</span></a></li></ol></li><li class="toc-item toc-level-2"><a class="toc-link" href="#%E6%95%B0%E6%8D%AE%E7%B1%BB%E5%9E%8B%E8%BD%AC%E6%8D%A2"><span class="toc-number">2.</span> <span class="toc-text">数据类型转换</span></a><ol class="toc-child"><li class="toc-item toc-level-3"><a class="toc-link" href="#int"><span class="toc-number">2.1.</span> <span class="toc-text">int()</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#float"><span class="toc-number">2.2.</span> <span class="toc-text">float()</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#str"><span class="toc-number">2.3.</span> <span class="toc-text">str()</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#bool"><span class="toc-number">2.4.</span> <span class="toc-text">bool()</span></a></li></ol></li><li class="toc-item toc-level-2"><a class="toc-link" href="#%E9%9B%86%E5%90%88%E7%9B%B8%E5%85%B3%E7%9A%84"><span class="toc-number">3.</span> <span class="toc-text">集合相关的</span></a><ol class="toc-child"><li class="toc-item toc-level-3"><a class="toc-link" href="#len"><span class="toc-number">3.1.</span> <span class="toc-text">len()</span></a></li></ol></li><li class="toc-item toc-level-2"><a class="toc-link" href="#%E5%AF%B9%E8%B1%A1%E7%9B%B8%E5%85%B3%E7%9A%84"><span class="toc-number">4.</span> <span class="toc-text">对象相关的</span></a><ol class="toc-child"><li class="toc-item toc-level-3"><a class="toc-link" href="#isinstance"><span class="toc-number">4.1.</span> <span class="toc-text">isinstance()</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#type"><span class="toc-number">4.2.</span> <span class="toc-text">type()</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#dir"><span class="toc-number">4.3.</span> <span class="toc-text">dir()</span></a></li></ol></li><li class="toc-item toc-level-2"><a class="toc-link" href="#%E9%AB%98%E9%98%B6%E5%87%BD%E6%95%B0"><span class="toc-number">5.</span> <span class="toc-text">高阶函数</span></a><ol class="toc-child"><li class="toc-item toc-level-3"><a class="toc-link" href="#map"><span class="toc-number">5.1.</span> <span class="toc-text">map</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#reduce"><span class="toc-number">5.2.</span> <span class="toc-text">reduce</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#filter"><span class="toc-number">5.3.</span> <span class="toc-text">filter</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#sorted"><span class="toc-number">5.4.</span> <span class="toc-text">sorted</span></a></li></ol></li><li class="toc-item toc-level-2"><a class="toc-link" href="#%E7%B1%BB%E4%B8%AD%E7%89%B9%E6%AE%8A%E7%9A%84%E6%96%B9%E6%B3%95"><span class="toc-number">6.</span> <span class="toc-text">类中特殊的方法</span></a><ol class="toc-child"><li class="toc-item toc-level-3"><a class="toc-link" href="#len-1"><span class="toc-number">6.1.</span> <span class="toc-text">__len__()</span></a></li></ol></li><li class="toc-item toc-level-2"><a class="toc-link" href="#%E7%B1%BB%E4%B8%AD%E7%89%B9%E6%AE%8A%E7%9A%84%E5%B1%9E%E6%80%A7"><span class="toc-number">7.</span> <span class="toc-text">类中特殊的属性</span></a><ol class="toc-child"><li class="toc-item toc-level-3"><a class="toc-link" href="#bases"><span class="toc-number">7.1.</span> <span class="toc-text">__bases__</span></a></li></ol></li></ol>
                </div>
            
            <hr />
            <div itemprop="articleBody"><h2 id="字符串"><a href="#字符串" class="headerlink" title="字符串"></a><strong>字符串</strong></h2><h3 id="lower"><a href="#lower" class="headerlink" title="lower()"></a><strong>lower()</strong></h3><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">&gt;&gt;&gt; </span><span class="built_in">print</span>(<span class="string">&#x27;ABC&#x27;</span>.lower())</span><br><span class="line">abc</span><br></pre></td></tr></table></figure>

<h2 id="数据类型转换"><a href="#数据类型转换" class="headerlink" title="数据类型转换"></a><strong>数据类型转换</strong></h2><h3 id="int"><a href="#int" class="headerlink" title="int()"></a><strong>int()</strong></h3><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">&gt;&gt;&gt; </span><span class="built_in">int</span>(<span class="string">&#x27;123&#x27;</span>)</span><br><span class="line"><span class="number">123</span></span><br><span class="line"><span class="meta">&gt;&gt;&gt; </span><span class="built_in">int</span>(<span class="number">12.34</span>)</span><br><span class="line"><span class="number">12</span></span><br></pre></td></tr></table></figure>

<h3 id="float"><a href="#float" class="headerlink" title="float()"></a><strong>float()</strong></h3><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">&gt;&gt;&gt; </span><span class="built_in">float</span>(<span class="string">&#x27;12.34&#x27;</span>)</span><br><span class="line"><span class="number">12.34</span></span><br></pre></td></tr></table></figure>

<h3 id="str"><a href="#str" class="headerlink" title="str()"></a><strong>str()</strong></h3><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">&gt;&gt;&gt; </span><span class="built_in">str</span>(<span class="number">1.23</span>)</span><br><span class="line"><span class="string">&#x27;1.23&#x27;</span></span><br><span class="line"><span class="meta">&gt;&gt;&gt; </span><span class="built_in">str</span>(<span class="number">100</span>)</span><br><span class="line"><span class="string">&#x27;100&#x27;</span></span><br></pre></td></tr></table></figure>

<h3 id="bool"><a href="#bool" class="headerlink" title="bool()"></a><strong>bool()</strong></h3><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">&gt;&gt;&gt; </span><span class="built_in">bool</span>(<span class="number">1</span>)</span><br><span class="line"><span class="literal">True</span></span><br><span class="line"><span class="meta">&gt;&gt;&gt; </span><span class="built_in">bool</span>(<span class="string">&#x27;&#x27;</span>)</span><br><span class="line"><span class="literal">False</span></span><br></pre></td></tr></table></figure>

<h2 id="集合相关的"><a href="#集合相关的" class="headerlink" title="集合相关的"></a><strong>集合相关的</strong></h2><h3 id="len"><a href="#len" class="headerlink" title="len()"></a><strong>len()</strong></h3><p>获取元素的个数：</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">&gt;&gt;&gt; </span><span class="built_in">len</span>([<span class="string">&#x27;lin&#x27;</span>, <span class="string">&#x27;cheng&#x27;</span>, <span class="string">&#x27;zhang&#x27;</span>])</span><br><span class="line"><span class="number">3</span></span><br></pre></td></tr></table></figure>

<h2 id="对象相关的"><a href="#对象相关的" class="headerlink" title="对象相关的"></a><strong>对象相关的</strong></h2><h3 id="isinstance"><a href="#isinstance" class="headerlink" title="isinstance()"></a><strong>isinstance()</strong></h3><p>判断对象的类型，如判断是否是一个可迭代对象：</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">&gt;&gt;&gt; </span><span class="keyword">from</span> collections.abc <span class="keyword">import</span> Iterable</span><br><span class="line"><span class="meta">&gt;&gt;&gt; </span><span class="built_in">print</span>(<span class="built_in">isinstance</span>(<span class="string">&#x27;abc&#x27;</span>, Iterable))  <span class="comment"># str是否可迭代</span></span><br><span class="line"><span class="literal">True</span></span><br><span class="line"><span class="meta">&gt;&gt;&gt; </span><span class="built_in">print</span>(<span class="built_in">isinstance</span>([<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>], Iterable))  <span class="comment"># list是否可迭代</span></span><br><span class="line"><span class="literal">True</span></span><br><span class="line"><span class="meta">&gt;&gt;&gt; </span><span class="built_in">print</span>(<span class="built_in">isinstance</span>(<span class="number">123</span>, Iterable))  <span class="comment"># 整数是否可迭代</span></span><br><span class="line"><span class="literal">False</span></span><br></pre></td></tr></table></figure>

<h3 id="type"><a href="#type" class="headerlink" title="type()"></a><strong>type()</strong></h3><p>查看对象的类型：</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">&gt;&gt;&gt; </span><span class="built_in">type</span>(<span class="number">123</span>)</span><br><span class="line">&lt;<span class="keyword">class</span> <span class="string">&#x27;int&#x27;</span>&gt;</span><br><span class="line"><span class="meta">&gt;&gt;&gt; </span><span class="built_in">type</span>(<span class="string">&#x27;str&#x27;</span>)</span><br><span class="line">&lt;<span class="keyword">class</span> <span class="string">&#x27;str&#x27;</span>&gt;</span><br><span class="line"><span class="meta">&gt;&gt;&gt; </span><span class="built_in">type</span>(<span class="literal">None</span>)</span><br><span class="line">&lt;<span class="built_in">type</span>(<span class="literal">None</span>) <span class="string">&#x27;NoneType&#x27;</span>&gt;</span><br><span class="line"><span class="meta">&gt;&gt;&gt; </span><span class="built_in">type</span>(<span class="built_in">abs</span>)</span><br><span class="line">&lt;<span class="keyword">class</span> <span class="string">&#x27;builtin_function_or_method&#x27;</span>&gt;</span><br></pre></td></tr></table></figure>

<h3 id="dir"><a href="#dir" class="headerlink" title="dir()"></a><strong>dir()</strong></h3><p>查询一个对象的所有属性和方法，包括私有属性和私有方法：</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">&gt;&gt;&gt; </span><span class="built_in">dir</span>(<span class="string">&#x27;ABC&#x27;</span>)</span><br><span class="line">[<span class="string">&#x27;__add__&#x27;</span>, <span class="string">&#x27;__class__&#x27;</span>,..., <span class="string">&#x27;__subclasshook__&#x27;</span>, <span class="string">&#x27;capitalize&#x27;</span>, <span class="string">&#x27;casefold&#x27;</span>,..., <span class="string">&#x27;zfill&#x27;</span>]</span><br></pre></td></tr></table></figure>



<h2 id="高阶函数"><a href="#高阶函数" class="headerlink" title="高阶函数"></a><strong>高阶函数</strong></h2><h3 id="map"><a href="#map" class="headerlink" title="map"></a><strong>map</strong></h3><p><code>map()</code> 函数接收两个参数：一个是函数，一个是 <code>Iterable</code> 对象。 <code>map</code> 将传入的函数依次作用到序列的每个元素，并把结果作为新的 <code>Iterator</code> 返回。</p>
<p>传入函数的特点：接收一个参数，返回一个值。</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">&gt;&gt;&gt; </span>L = [<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>, <span class="number">4</span>, <span class="number">5</span>, <span class="number">6</span>, <span class="number">7</span>, <span class="number">8</span>, <span class="number">9</span>, <span class="number">10</span>]</span><br><span class="line"><span class="meta">&gt;&gt;&gt; </span><span class="built_in">print</span>(<span class="built_in">list</span>(<span class="built_in">map</span>(<span class="keyword">lambda</span> x: x * x, L)))</span><br><span class="line">[<span class="number">1</span>, <span class="number">4</span>, <span class="number">9</span>, <span class="number">16</span>, <span class="number">25</span>, <span class="number">36</span>, <span class="number">49</span>, <span class="number">64</span>, <span class="number">81</span>, <span class="number">100</span>]</span><br></pre></td></tr></table></figure>

<h3 id="reduce"><a href="#reduce" class="headerlink" title="reduce"></a><strong>reduce</strong></h3><p><code>reduce()</code> 函数接收两个参数：一个是函数，一个是 <code>Iterable</code> 对象。 <code>reduce</code> 把结果继续和序列的下一个元素做累积计算，其效果就是：</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">from</span> functools <span class="keyword">import</span> reduce</span><br><span class="line">reduce(f, [x1, x2, x3, x4]) = f(f(f(x1, x2), x3), x4)</span><br></pre></td></tr></table></figure>

<p>传入函数的特点：接收两个参数，返回一个值。</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">&gt;&gt;&gt; </span><span class="keyword">from</span> functools <span class="keyword">import</span> reduce</span><br><span class="line">&gt;&gt;&gt;</span><br><span class="line"><span class="meta">&gt;&gt;&gt; </span>L = [<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>, <span class="number">4</span>, <span class="number">5</span>, <span class="number">6</span>, <span class="number">7</span>, <span class="number">8</span>, <span class="number">9</span>, <span class="number">10</span>]</span><br><span class="line"><span class="meta">&gt;&gt;&gt; </span><span class="built_in">print</span>(reduce(<span class="keyword">lambda</span> x, y: x + y, L))</span><br><span class="line"><span class="number">55</span></span><br></pre></td></tr></table></figure>

<h3 id="filter"><a href="#filter" class="headerlink" title="filter"></a><strong>filter</strong></h3><p><code>filter()</code> 函数接收一个函数和一个序列。<code>filter()</code> 把传入的函数依次作用于每个元素，然后根据返回值 <code>True/False</code> 决定是否保留该元素。</p>
<p>传入函数的特点：接收一个参数，返回 <code>True/False</code> 。</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">&gt;&gt;&gt; </span>L = [<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>, <span class="number">4</span>, <span class="number">5</span>, <span class="number">6</span>, <span class="number">7</span>, <span class="number">8</span>, <span class="number">9</span>, <span class="number">10</span>]</span><br><span class="line"><span class="meta">&gt;&gt;&gt; </span><span class="built_in">print</span>(<span class="built_in">list</span>(<span class="built_in">filter</span>(<span class="keyword">lambda</span> x: x % <span class="number">2</span> == <span class="number">0</span>, L)))</span><br><span class="line">[<span class="number">2</span>, <span class="number">4</span>, <span class="number">6</span>, <span class="number">8</span>, <span class="number">10</span>]</span><br></pre></td></tr></table></figure>

<h3 id="sorted"><a href="#sorted" class="headerlink" title="sorted"></a><strong>sorted</strong></h3><p><code>sorted()</code> 函数接收三个参数：序列，key，reverse。其中 key 是个默认参数，接收一个映射函数。reverse 也是个默认参数，传入 <code>True/False</code> 表示是否反转排序后的序列。</p>
<p><code>sorted()</code> 函数默认按从小到大排序。</p>
<p>传入函数的特点：接收一个参数，返回 <code>int</code> 值。</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">&gt;&gt;&gt; </span>L = [-<span class="number">5</span>, <span class="number">4</span>, <span class="number">2</span>, -<span class="number">2</span>, <span class="number">1</span>, <span class="number">0</span>, -<span class="number">1</span>, <span class="number">5</span>, -<span class="number">4</span>, -<span class="number">3</span>, <span class="number">3</span>]</span><br><span class="line"><span class="meta">&gt;&gt;&gt; </span><span class="built_in">print</span>(<span class="built_in">sorted</span>(L))</span><br><span class="line">[-<span class="number">5</span>, -<span class="number">4</span>, -<span class="number">3</span>, -<span class="number">2</span>, -<span class="number">1</span>, <span class="number">0</span>, <span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>, <span class="number">4</span>, <span class="number">5</span>]</span><br><span class="line"><span class="meta">&gt;&gt;&gt; </span><span class="built_in">print</span>(<span class="built_in">sorted</span>(L, key=<span class="keyword">lambda</span> x: x * x))</span><br><span class="line">[<span class="number">0</span>, <span class="number">1</span>, -<span class="number">1</span>, <span class="number">2</span>, -<span class="number">2</span>, -<span class="number">3</span>, <span class="number">3</span>, <span class="number">4</span>, -<span class="number">4</span>, -<span class="number">5</span>, <span class="number">5</span>]</span><br><span class="line"><span class="meta">&gt;&gt;&gt; </span><span class="built_in">print</span>(<span class="built_in">sorted</span>(L, reverse=<span class="literal">True</span>))</span><br><span class="line">[<span class="number">5</span>, <span class="number">4</span>, <span class="number">3</span>, <span class="number">2</span>, <span class="number">1</span>, <span class="number">0</span>, -<span class="number">1</span>, -<span class="number">2</span>, -<span class="number">3</span>, -<span class="number">4</span>, -<span class="number">5</span>]</span><br></pre></td></tr></table></figure>

<h2 id="类中特殊的方法"><a href="#类中特殊的方法" class="headerlink" title="类中特殊的方法"></a><strong>类中特殊的方法</strong></h2><p>类似 <code>__xxx__</code> 方法在 Python 中都是有特殊用途的。</p>
<h3 id="len-1"><a href="#len-1" class="headerlink" title="__len__()"></a>__len__()</h3><p><code>__len__()</code>  方法返回对象的长度。调用 <code>len()</code> 函数获取一个对象的长度，实际上，在 <code>len()</code> 函数内部，它自动去调用该对象的 <code>__len__()</code> 方法。所以，下面的代码是等价的：</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">&gt;&gt;&gt; </span><span class="built_in">len</span>(<span class="string">&#x27;ABC&#x27;</span>)</span><br><span class="line"><span class="number">3</span></span><br><span class="line"><span class="meta">&gt;&gt;&gt; </span><span class="string">&#x27;ABC&#x27;</span>.__len__()</span><br><span class="line"><span class="number">3</span></span><br></pre></td></tr></table></figure>

<h2 id="类中特殊的属性"><a href="#类中特殊的属性" class="headerlink" title="类中特殊的属性"></a><strong>类中特殊的属性</strong></h2><p>类似 <code>__xxx__</code> 属性在 Python 中都是有特殊用途的。</p>
<h3 id="bases"><a href="#bases" class="headerlink" title="__bases__"></a><strong>__bases__</strong></h3><p>当一个对象表示数据类型时，比如 int 对象，它就具有了 <code>__bases__</code> 属性：</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">&gt;&gt;&gt; </span><span class="built_in">print</span>(<span class="built_in">int</span>.__bases__)</span><br><span class="line">(&lt;<span class="keyword">class</span> <span class="string">&#x27;object&#x27;</span>&gt;,)</span><br></pre></td></tr></table></figure>





























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